Most Indian customers who ask for a battery want one thing: the fans, lights and Wi-Fi keep running when the power goes. Some also want the fridge, the TV, a water pump or even an AC. The battery size depends almost entirely on which loads and for how many hours, so the most important sizing tool is a patient conversation and a notepad. Get the load list right and the arithmetic is easy.
Step 1: Agree the essential loads
Ask the customer to walk you through a typical power cut. When does it happen? How long does it last? What do they actually switch on?
Write down each load, its power, how many, and how many hours it runs during the outage. Use the nameplate or a plug-in power meter, not guesses.
Typical figures (check the actual appliances):
| Load | Typical power |
|---|---|
| Ceiling fan, conventional | 70–80 W |
| Ceiling fan, BLDC | 28–35 W |
| LED tube or bulb | 9–20 W |
| Wi-Fi router and ONT | 10–20 W |
| LED TV, 43 inch | 80–120 W |
| Refrigerator, 250 L | 100–200 W running, about 1–1.5 kWh/day |
| Laptop | 45–65 W |
| 1 hp water pump | 750 W running, several times that at start |
| 1.5 ton inverter AC | 1.0–1.6 kW running |
Fridges cycle. The compressor runs only part of the time, so use its daily energy (from the star label) divided by 24 for the average power, and its running power for the peak.
Step 2: Energy for the outage
Worked example: a family in Lucknow, 4-hour evening cuts
| Load | Qty | Power | Hours | Energy |
|---|---|---|---|---|
| Ceiling fans | 4 | 75 W | 4 | 1.20 kWh |
| LED lights | 6 | 10 W | 4 | 0.24 kWh |
| Fridge (average) | 1 | 60 W | 4 | 0.24 kWh |
| Wi-Fi router | 1 | 15 W | 4 | 0.06 kWh |
| LED TV | 1 | 100 W | 2 | 0.20 kWh |
| Total | 1.94 kWh |
Notice that the fans dominate. Replacing four 75 W fans with 30 W BLDC fans saves 0.72 kWh per outage, more than a third of the battery. Sometimes the cheapest battery advice is "change the fans".
Step 3: Peak power and surge
Add up everything that could run at the same time: 300 + 60 + 150 (fridge running) + 15 + 100 = 625 W. A fridge compressor can draw several times its running power for a second when it starts, so the inverter must handle a short surge of perhaps 1.2–1.5 kW.
- Continuous rating: at least the simultaneous load with margin. 625 W suits a 1 kVA inverter, but on a solar project the hybrid inverter will usually be 3–5 kW anyway for the PV.
- Surge rating: check the datasheet (often 1.5–2 times rated for a few seconds).
- Motor loads: a 1 hp pump or an AC can trip a small inverter at start. Either keep them off the backup circuit or size the inverter for them and use a soft starter or inverter-type AC.
Step 4: From delivered energy to battery size
The battery must supply the load energy plus the inverter's losses, without going past its allowed DoD, and still do so when it has aged.
Nominal battery kWh = Load kWh ÷ inverter efficiency ÷ DoD ÷ ageing factor
Option A: LFP
- Inverter efficiency on battery: about 0.92
- DoD: 0.9
- Ageing: design for 80% capacity at end of life, factor 0.8
1.94 ÷ 0.92 ÷ 0.9 ÷ 0.8 = 2.93 kWh nominal
A 51.2 V 60 Ah pack (3.07 kWh) fits. In practice many customers choose a standard 5 kWh pack, which also gives room for longer cuts.
Current check: 625 W ÷ 51.2 V ÷ 0.92 = 13 A, well within a 60 Ah pack's 0.5C rating (30 A).
Option B: tubular lead-acid at 24 V
- Inverter efficiency: about 0.90
- DoD: 0.5 for regular use
- Ageing allowance: 0.8
1.94 ÷ 0.9 ÷ 0.5 ÷ 0.8 = 5.39 kWh nominal
At 24 V: 5,390 ÷ 24 = 225 Ah, so two 12 V tubular batteries of at least 225 Ah in series, for example 230 Ah (5.52 kWh). A 220 Ah pair (5.28 kWh) would be slightly short.
Current check: 625 ÷ 24 ÷ 0.9 = 29 A, about C/8: fine for a C10-rated battery.
The lead-acid bank is nearly twice the nominal size of the LFP pack for the same backup, and four times the weight.
Step 5: Can the solar recharge it?
Backup only works if the battery is full when the next cut comes. With evening cuts, the PV must recharge the battery during the day in addition to running daytime loads.
For the LFP option: 1.94 ÷ 0.92 = 2.1 kWh out of the battery, plus about 5% charging loss: about 2.2 kWh to recharge. On a clear winter day in Lucknow a 3 kWp array produces roughly 9–10 kWh (the December average is only about 3 units per kWp a day). That is plenty, even with daytime loads, but in a week of monsoon cloud it may not be. Most hybrid inverters can top up from the grid when solar is short; set that up deliberately.
Long and irregular outages
In many rural and semi-urban areas cuts are long and unpredictable: 6–10 hours, sometimes overnight. Two approaches:
- Size for the realistic worst night and accept a larger battery.
- Prioritise loads: wire a separate "essential" circuit for lights, a fan per room and the router, and let the hybrid inverter shed the rest when the battery falls below, say, 40%.
Key points
- Battery size is driven by the essential-load list and the outage hours; spend time getting them right.
- Energy (kWh) sets the battery; simultaneous power and motor surges set the inverter.
- Nominal kWh = load kWh ÷ inverter efficiency ÷ DoD ÷ ageing factor.
- For the same backup, lead-acid needs about twice the nominal kWh of LFP because of its lower usable DoD.
- Check that solar (or the grid) can recharge the battery before the next outage.
- Efficient appliances, like BLDC fans, can shrink the battery more cheaply than buying more kWh.
Check yourself
- Loads: 3 fans at 75 W and 4 lights at 12 W for 5 hours. What energy is needed?
- Size an LFP battery for that load (inverter 0.92, DoD 0.9, ageing 0.8).
- Why does the inverter need a surge rating higher than the running load?
Answers
- (225 + 48) W × 5 h = 1,365 Wh = 1.37 kWh.
- 1.37 ÷ 0.92 ÷ 0.9 ÷ 0.8 ≈ 2.07 kWh nominal; for example a 51.2 V 50 Ah pack (2.56 kWh).
- Motors and compressors draw several times their running current for a moment when they start; without enough surge capacity the inverter trips.